Implantable Medical Devices (IMDs) for producing a therapeutic result in a patient are well known. For example, implantable neurostimulators are available for the treatment of pain, movement disorders such as Parkinson's disease, essential tremor, dystonia, gastric disorders, incontinence, sexual disfunction, migraine headaches, and other conditions. Other examples of IMDs include, but are not limited to, implantable drug infusion pumps, cardioverters, cardiac pacemakers, defibrillators, and cochlear implants.
All of the foregoing types of IMDs require electrical power to perform their therapeutic function, which may include driving an electrical infusion pump, providing an electrical neurostimulation pulse, or providing an electrical cardiac stimulation pulse. This electrical power is derived from a power source.
Typically, a power source for an IMD can take one of two forms. The first form utilizes an external power source that transcutaneously delivers energy via wires or radio frequency energy. However, having electrical wires that perforate the skin is disadvantageous due, in part, to the risk of infection. Further, continuously coupling patients to an external power source for therapy is a large inconvenience.
The second type of power source utilizes single cell batteries to provide energy to the IMD. This can be effective for low-power applications such as pacing devices. However, such single cell batteries usually do not supply the lasting power required to perform therapies provided by newer IMDs. In some cases, such as an implantable artificial heart, a single cell battery might last the patient only a few hours. In other, less extreme cases, a single cell unit might expel all or nearly all of its energy in less than a year. This will necessitate the explant and re-implant of the IMD.
One mechanism that addresses the foregoing limitations allows electrical power to be transcutaneously transferred through the use of inductive coupling. The transferred electrical power can optionally be stored in a rechargeable battery. This battery can then be used to provide direct electrical power to the IMD. When the battery has expended, or nearly expended, its capacity, the battery can be recharged transcutaneously.
Transcutaneous energy transfer through the use of inductive coupling involves the placement of two coils positioned in close proximity to each other on opposite sides of the skin (i.e., cutaneous boundary). One of these coils is external to the patient, and is placed against the patient's skin in the vicinity of the IMD. This external, or primary, coil is associated with an external power source or external charger or recharger. A secondary coil is implanted within the patient, and may be part of the IMD or otherwise associated therewith.
In one embodiment, the primary coil is driven by the external power source with an alternating current. This induces a current in the secondary coil through inductive coupling. This induced current may be used to power the IMD and/or to charge or recharge an internal power source.
For IMDs, the efficiency at which energy is transcutaneously transferred may be crucial for several reasons. First, the inductive coupling has a tendency to heat surrounding components and tissue. Since it is desirable to limit this heating effect, the amount of energy transfer per unit time must also be limited. The higher the efficiency of energy transfer, the more energy that can be transferred while at the same time limiting the heating of surrounding tissue.
In addition to the foregoing, it is desirable to limit the amount of time required to achieve a desired charge, or recharge, of an internal power source. While charging or recharging is occurring, the patient necessarily has an external encumbrance attached to his or her body. This attachment may impair the patient's mobility and limit the patient's comfort. The higher the efficiency of the energy transfer system, the faster the desired charging or recharging can be completed, thus limiting inconvenience to the patient.
Finally, the amount of energy available to the IMD may be limited by the amount of time the patient is willing to devote to recharging the device. The higher the efficiency of the energy transfer system, the greater the amount of energy that can be transferred during the recharge time. This increases the practical size of the internal power source, and allows for use of IMDs having higher power use requirements. This may also extend the time between charging.
One way to increase the efficiency of energy transfer is to position the primary coil optimally with respect to the secondary coil. Some existing implantable medical device systems incorporate a locating feature to locate, and align, the secondary coil with respect to the primary coil. This feature has variously been implemented using a metal detection approach that involves measuring the loading of the external antenna caused by the proximity of the implant. When the loading is at a maximum, a high-efficiency coupling has been achieved. However, because the loading of the system is affected by non-device metallic objects in the vicinity of the device, false results can be produced, resulting in non-optimal alignment.
Other solutions use power measurements in the IMD to determine the efficiency of the energy transfer. In such systems, a recharge session is initiated. After the recharge session begins, one or more sensors in the IMD measure current in, and/or voltage across, the secondary coil. Periodically, the recharge session is temporarily halted so that the IMD may transfer the measurements to the recharging unit. These measurements may then be used to determine the efficiency of the energy transfer. For instance, the power associated with the primary coil may be compared to that measured in the secondary coil to determine the efficiency of the power transfer. If the efficiency is not adequate, the position of the primary coil may be moved with respect to the IMD and the process is repeated.
As a variation of the foregoing, current associated with the secondary coil may be measured. If the current induced in the secondary coil is above some minimum required current, the coupling efficiency is considered adequate. Otherwise, the primary coil must be repositioned and the process repeated.
The foregoing mechanisms of measuring electrical characteristics of a secondary coil during recharge do not occur in real time. A patient or clinician must position the primary coil at some location relative to the IMD and initiates a recharge session. Sometime thereafter, this recharge session is interrupted so that measurements taken during recharge may be transferred from the IMD to the recharging unit. One or more calculations to determine coupling efficiency may be performed and the results are communicated to the patient. If the efficiency is not adequate, the primary coil is repositioned, recharge is re-initiated, and the process is repeated. A single iteration may take a better part of a minute. Thus, finding the optimal location may take a substantial amount of time.